The present invention concerns a method for cool-drying gas containing water vapor, whereby this gas is guided through the secondary part of a heat exchanger whose primary part is the evaporator of a cooling circuit which also contains a compressor which is driven by an electric motor, a condenser, an expansion means between the outlet of the condenser and the inlet of the evaporator, and whereby the above-mentioned cooling circuit is thus controlled as a function of the load that the cooling capacity is adjusted without any ice being formed in the evaporator.
Such methods are used among others for drying compressed air.
Compressed air which is supplied by a compressor is in most cases saturated with water vapor or, in other words, has a relative humidity of 100%. This implies that there is condensation at the slightest decrease of temperature. The water of condensation causes corrosion in the pipes and tools, and the equipment will wear out prematurely.
That is why the compressed air is dried, which may be done in the above-mentioned manner, by means of cool-drying. Also other air than compressed air or other gases may be dried in this manner.
Cool-drying is based on the principle that, by lowering the temperature, moisture from the air or the gas condenses, after which the water of condensation is separated in the liquid separator and after which the air or the gas is heated again, as a result of which this air or this gas is no longer saturated. The heat is discharged by the cooling circuit in the evaporator.
The same applies to other gases than air, and each time air is referred to hereafter, the same also applies to other gases than air.
In practice, there is an ISO-standard which determines the possible dew point and the corresponding lowest air temperature for reference values.
In order to prevent the lowest air temperature from dropping below 0xc2x0 C. and thus the evaporator from freezing up, a necessary condition is that the temperature of the evaporator is higher than 0xc2x0 C.
According to known methods, to this end, the temperature is measured on the inlet of the evaporator, or, since there is a definite connection between the temperature of the evaporator and the pressure of the evaporator for a specific cooling liquid in the cooling circuit, the pressure is measured before or after the evaporator.
The cooling circuit is then controlled such that the temperature of the evaporator or the pressure of the evaporator has the required value, and for example the pressure of the evaporator coincides with a temperature which is situated a few degrees below the required lowest air temperature or LAT, but not below 0xc2x0 C.
According to these known methods for cool-drying, the motor of the compressor of the cooling circuit which is driven at a constant frequency is switched on and off as a function of the temperature of the evaporator. If this pressure of the evaporator decreases too much, said motor is stopped. If the pressure of the evaporator subsequently increases too much as the expansion valve is still open, the motor is started again.
Such a regulation makes it possible for the compressor to be switched off when the load drops beneath the cooling capacity, as a result of which the energy consumption will decrease. The surplus of cooling capacity is stored in a thermal mass. However, this regulation is very disadvantageous as the compressor is continuously switched on and off in case of a small load, while also the pressure of the evaporator and the dew points fluctuate strongly. Moreover, the cool-dryer must be built relatively large.
Another known method consists in measuring the lowest air temperature (LAT) on the outlet of the secondary part of the heat exchanger and to switch off the motor of the compressor of the cooling circuit when the temperature threatens to drop below 0xc2x0 C. This method, whereby the motor is thus also switched on and off, offers the same disadvantages as the preceding one.
Another possibility for regulating the pressure of the evaporator would consist in selecting an evaporator which is large enough and in carrying back hot gases on the outlet of the compressor to the inlet of the compressor by means of a bypass.
This regulation method is disadvantageous in that, since the compressor motor is continuously working, also when there is no load or when the load is low, the energy consumption is equal to the energy consumption at a nominal load, as the high and low pressure in the cooling circuit are continuously kept at a constant level.
The object of the invention is a method for cool-drying which does not have the above-mentioned and other disadvantages and which makes it possible to save energy in a simple manner, without any pressure variations in the cooling circuit and without much wear of the compressor and its motor.
In accordance with the invention, this object is accomplished in that the cooling circuit is controlled by adjusting the rotational speed of the motor.
Instead of switching the motor on or off, its speed is adjusted. By increasing the rotational speed of the motor, more mass flow of cooling liquid can be pumped round, and thus can be obtained a higher cooling output.
The temperature of the evaporator can be measured and the above-mentioned cooling circuit can be controlled as a function of the measured temperature of the evaporator.
According to another embodiment, the pressure of the evaporator can be measured and the above-mentioned cooling circuit can be controlled as a function of the measured pressure of the evaporator.
According to yet another embodiment, the lowest gas temperature (LAT) can be measured and the above-mentioned cooling circuit is controlled as a function of this lowest gas temperature (LAT).
According to yet another embodiment, the dew point temperature of the gas can be measured and the above-mentioned cooling circuit is controlled as a function of this dew point.
Preferably, the rotational speed of the motor is adjusted by modifying the frequency of the supply current.
According to a special embodiment of the invention, the ambient temperature is measured and the rotational speed of the motor is adjusted as a function of the measured ambient temperature.
At high ambient temperatures, whereby the air or the gas is also relatively warm and may contain more moisture than when it is cold, it is not necessary to cool it to 3xc2x0 C. in the heat exchanger in order to obtain dry air. Thus, the energy consumption of the above-mentioned cool-dryers is too high, and they require relatively large and expensive components in order to supply the cooling output. By taking into account said ambient temperature, the required cooling output may be kept lower, such that the cool-dryer can be made less large.
Preferably, the rotational speed of the motor of the compressor is adjusted such that the lower air or gas temperature on the outlet of the evaporator is 20xc2x0 C. lower than the measured ambient temperature, without dropping below 3xc2x0 C., however.
It is assumed that, when the outgoing air or the outgoing gas has a relative humidity of 50%, the danger of corrosion in pipes and equipment is excluded, and the above-mentioned control device guarantees that said relative humidity will not be higher than 50%.
The invention also concerns a device for cool-drying or a cool-dryer which is particularly suitable for applying the above-mentioned method.
The invention in particular concerns a device for cool-drying, containing a heat exchanger whose primary part is the evaporator of a cooling circuit which also contains a compressor which is driven by an electric motor, a condenser, an expansion means between the outlet of the condenser and the inlet of the evaporator, a control device to control the above-mentioned motor and measuring means coupled thereto, whereas the secondary part of the heat exchanger is part of a pipe for the gas and a liquid separator is erected on the outlet of said heat exchanger, in said pipe, whereby the device contains means to adjust the rotational speed of the motor while the control device controls these means as a function of the value measured by the measuring means.
The measuring means may be provided on the cooling circuit and they may be means to measure the temperature of the evaporator or the pressure of the evaporator.
However, the measuring means may also be provided on the pipe for the gas, in the secondary part of the heat exchanger or downstream to it, and they may be means to measure the lowest gas temperature (LAT) or means to measure the dew point.
Preferably, the means for regulating the rotational speed of the motor consist of a frequency converter.
According to a special embodiment of the invention, the cool-dryer contains means for measuring the ambient temperature which are also coupled to the control device, and this control device is such that it adjusts the speed of the motor both as a function of the value measured by the measuring means and as a function of the temperature measured by the means for measuring the ambient temperature.